It's dark, the only light a fat streak of silver across the floor. It's a full moon, and on the full moon when evil isn't threatening Beacon Hills, Stiles and Derek tend to stay in, watch a movie, and sleep.

It's the one night a month they've never had full sex, and Stiles joked that he might as well have been dating a girl for that kind of regularity.

Tonight is different.

They've been talking about it for months, but the way Beacon Hills draws dark things, the timing was never right. If they got to bed at all, they were battered and bruised, or too tired to attempt it.

Tonight it's quiet. There's nothing lurking in the shadows. The pack is spread out, in their own homes, enjoying a quiet evening for a change. Stiles and Derek have the loft to themselves, and the timing is perfect.

"Are you sure?" Derek whispers, feeling as though the calm might be broken at any moment.

Stiles turns to look over his shoulder. "For the fiftieth time, I'm sure." He wiggles his ass and pushes back into Derek's hands. "Knot me already."

Derek lets out a huff of laughter and drags his cock down the crack of Stiles' ass. He's half-hard, any more and he won't be able to get in. Not tonight.

Because werewolves knot on the full moon.

Just one night a month when their physiology is so removed from human it's obvious why most werewolves pair off with their own kind. Human bodies are fragile, and the drive to mate is strong. It's only through a lifetime of control that Derek could even share the same bed as Stiles on the full moon.

Still, they could barely touch each other.

There have been nights when that control slipped, when someone reached out to the other, and Derek wanted so badly to be inside, to pump his hips into tight heat, but he held back.

It's because of that Stiles knows what he's getting into. He knows how big the knot can get, how long it takes to go down. He's felt it in his hands, he's taken it between his thighs—and come out bruised and sore even without penetration. He knows how full he's going to be, and he knows that he'll be that way for most of the night.

With one last, deep breath, and clinging to every ounce of restraint he has, Derek pushes into Stiles' body.

It's easy, until he sinks deep enough that Stiles' rim starts to stretch around his partially swollen knot. His cock hardens fast, the knot swells further, and he's got to hold Stiles hard by the hips to get in.

Stiles cries out. Derek's hurting him, he knows that, but Stiles knew what would happen, knew what struggling would do, and to his credit he doesn't try to pull away. Then Derek's inside, and he sighs as his cock continues to swell, until he's fully hard and locked tight inside Stiles' body.

Stiles is shaking, breathing hard, back arched as his body fights the intrusion.

"Shh," Derek forces out. "You're okay." It's all he can manage before he's overcome, as his orgasm builds and builds and washes away all that's human.

He can't think past the heat, the tightness, the fact that it's the first time he's ever knotted inside someone. He feels it in his balls, an intense drawing as if he's being turned inside out by increments. He's come on a full moon before, of course he has, but it's never been like this. It's never felt so right, never before has he been without that nagging instinct that told him he was wasting it, he was doing it wrong.

"Holy fuck, Derek," Stiles says. He's shaking, sweat trickling down his spine to pool in the small of his back.

Derek brings his fingers to his lips to lick the salt of Stiles' skin from his claws. He can't speak, all he knows is that he's filling his mate, breeding him. There's a tiny part of his mind that reminds him that's impossible, but instinct doesn't care about that. He keeps his hands on Stiles though, stroking the length of his back, his sides, keeping up a soft, low growl throughout that means 'safe' and 'mine'.

"So full, Derek," Stiles breathes, his voice raw and wrecked. "Too much, I can't—" He writhes, tries to pull away and cries out.

Derek lets out a harsher growl, short, sharp, bitten off, and drops down over Stiles' back. He holds Stiles' there, because with Derek's arms over his shoulders, he's got no room to move. "Stay," he manages, and he knows Stiles would be making a dog joke if he could.

"You just keep coming," Stiles whimpers. "I'm so full." He shifts, reaching beneath himself. "Oh god."

Derek whines and finds Stiles' hand with his own. It's pressed against his stomach, rounded and tight, becoming distended as Derek fills him.

"You're gonna stop soon, right?" Stiles says, his voice a high pitched plea. "'Cause I don't think I can take anymore."

Derek grunts an affirmative, feeling it ease already, feeling himself coming back from a place where he runs purely on instinct. "Stiles," he breathes, low and heavy. "Stiles."

"I'm here." Stiles breathes out, long and slow. His body is humming, shivers rocking him every few seconds. "How did we not think about the whole 'where's it gonna go?' issue? I knew there'd be a lot, but..." He turns his head, looks over his shoulder, and his eyes are wide. "It feels weird."

Derek drops his head onto Stiles' back, presses his face into sweat-slick skin and spans his hand out over Stiles' swollen belly. "Mine," he grunts, and wraps his other arm around Stiles' chest.

Stiles drops his head and rolls his hips. "That's right," he breathes. "I'm yours."

Derek rubs his face on Stiles' skin and lets it out again, the low pitched growl that means 'home' and 'happy'.

fin